Technical Abstract:
In Mexico, although Listeria monocytogenes is not a mandatory diagnostic pathogen, neither in food nor in suspected clinical cases, the bacterium has been recovered from food. The latter highlights the importance of further characterizing the comparative virulence properties L. monocytogenes recovered from foods and associated clinical cases to provide knowledge for understanding its importance as a foodborne pathogen in Mexico. In the present study, twenty-three L. monocytogenes strains isolated from food sources belonging to five serotypes (1/2a, 1/2b, 1/2c, 3b and 4b) were characterized by the adhesion pattern and invasion level in HEp-2 and HT-29 cells in the presence of D-mannose. Strains that adhered to the monolayers were classified as localized, diffuse, or aggregative patterns. Regardless of the food origin and serotype of isolates, all 23 L. monocytogenes strains adhered to HEp-2 and HT-29 cells. The adherence patterns observed were diffuse with respect to the criteria used to describe adherence for the well characterized bacterium Escherichia coli. We also observed the formation of vacuoles, cell-to-cell extensions, and cellular damage, suggesting a cytotoxic effect. The diffuse distribution observed can lead L. monocytogenes to invade the epithelial cells and this property was quantified as follows: the rate to invade HT-29 by HEp-2 cells of these strains ranged from 3.24 to 5.35 log CFUmL-1 and from 3.49 to 4.84 log CFUmL-1, respectively. Statistical differences were observed among strains and cell lines (p=0.000). Thus, although in Mexico there are not many clinical reports, our results support the presence of virulent strains associated with foods that presumably are involved in clinical cases. Further studies are required to identify and compare clinical and food strains in different cell types to define a clear association among the adherent phenotype and invasion capacity with a clinical manifestation.